tests: Port to glib-tap.mk, make make check run all of the tests

OSTree's code for testing predates the `glib-tap.mk` making its
way into GLib.  Let's switch to it, as it provides a number
of advantages.

By far the biggest advantage is that `make check` can start to run
most of the tests *in addition* to having them work installed.

This commit keeps the installed tests working, but `make check` turns
out to be really broken because...our TAP usage has bitrotted to say
the least.  Fix that all up.

Do some hacks so that the tests work uninstalled as well - in
particular, `glib-tap.mk` and the bits encoded into
`g_test_build_filename()` assume *recursive* Automake (blah).  Work
around that by creating a symlink when installed to loop back.

tests/gpghome/pubring.gpg

@@ -0,0 +1 @@
+trusted/pubring.gpg